The field is internal combustion engines. Particularly the field is related to constructions for thermal management of pistons. In some aspects, the field includes internal combustion engines in which the end surface of a piston crown is insulated from the ring area of the piston. In some other aspects, the field includes high compression diesel engines, particularly opposed-piston diesel engines.
During operation of an internal combustion engine, combustion of an air/fuel mixture occurs in cylinder space defined by the end surface on the crown of at least one piston reciprocating in the cylinder. For example, in an opposed-piston engine, combustion occurs in the cylinder space defined between the end surfaces on the crowns of two opposed pistons near respective top dead center positions in a cylinder. The heat of air compressed between the end surfaces of the crowns causes fuel injected into the heated air to burn. The cylinder space where fuel combusts is typically referred to as a “combustion chamber”.
In order to maximize the conversion of the energy released by combustion into motion, it is desirable to prevent heat from being conducted away from the combustion chamber through the piston. Reduction of heat lost through the piston increases the engine's operating efficiency. Typically, heat transfer through the piston is reduced or blocked by insulating the piston crown from the body of the piston. However, it is also the case that retention of the heat of combustion at the end surface of the piston can cause thermal damage to the piston crown and nearby piston elements.
Piston thermal management is a continuing problem, especially given the ever-increasing loads expected from modern internal combustion engines. In a typical piston, at least four areas are of concern for thermal management: the piston crown, the ring grooves, the piston under-crown, and the piston/wristpin interface. The piston crown can be damaged by oxidation if its temperature rises above the oxidation temperature of the materials of which it is made. Mechanical failure of piston elements can result from thermally-induced material changes. The rings and ring grooves and the lands that border the ring grooves can suffer from carbon build-up caused by oil heated above the coking temperature. As with the ring grooves the under surface of the piston crown can also suffer from oil coking.
A recent study indicates that an opposed-piston engine two-stroke cycle engine exhibits increased thermal efficiency when compared with a conventional six-cylinder four-cycle engine. (Herold, R., Wahl, M., Regner, G., Lemke, J. et al., “Thermodynamic Benefits of Opposed-Piston Two-Stroke Engines,” SAE Technical Paper 2011-01-2216, 2011, doi:10.4271/2011-01-2216.) The opposed-piston engine achieves thermodynamic benefits by virtue of a combination of three effects: reduced heat transfer due to a more favorable combustion chamber area/volume ratio, increased ratio of specific heats from leaner operating conditions made possible by the two-stroke cycle, and decreased combustion duration achievable at the fixed maximum pressure rise rate arising from the lower energy release density of the two-stroke engine. With two pistons per cylinder, an opposed-piston engine can realize additional thermodynamic benefits with enhanced piston thermal management.